The disclosure relates generally to electrophotographic printer systems, and more particularly the dynamic focusing of an exposure device using piezoelectric stacks mounted on ether end of an LED bar to adjust the focus of the bar against the photoreceptor surface.
Print bar type imager assemblies consist of an array, usually linear, of individual sources. These print bars are typically made up of smaller sub-arrays butted side by side to make a longer array. A “print bar” as used in this document means a structure or device holding an arrangement of printheads that remains stationary during printing. For print bars or printheads the prevalent technology currently is the light emitting diode (“LED”) bar. A lens mechanism such as a rod lens array (commercially available under the trademarked name SELFOC) can be used in the print bar for focusing the light emitted by the LEDs on the photosensitive recording member such as a photoreceptor (P/R) medium. However, due to the limitations and tolerances of the lens mechanism, the depth of focus of a Selfoc lens is very small. Depth of focus is the tolerance in which either the light source, the Selfoc lens or photoreceptor can have a positional error (around ±60 μm) with respect to the other two without losing the focus. Moving out of this focus range results in imaging defects, see FIG. 3. To maintain this mechanical tolerance (around ±60 μm) or to bring the system back in can require adjustment due to production variations, environmental changes or wear over life. This constant adjustment adds to design and production cost. Various techniques have been proposed to address the so called depth of focus problem in electrophotographic printing. Depth of focus correcting techniques have included replacing the light source with a laser, changing the spot size by eliminating the lens mechanism, replacing the light source, and software processing to change the illumination profile of the light source.
There is a need in the art for methods and systems that can optimally control the position of the printbar or printhead to correct for process variations and other factors that may adversely affect the depth of focus or positional errors when forming an image on a photoreceptor medium.